The present invention relates generally to a control system for electrostatic precipitator rappers. More particularly, the invention relates to such a system which includes means for accurately indicating rapper plunger lift and therefore rapping intensity.
Electrostatic precititators are widely employed, particularly among industrial users, for removing particulate from gases. A typical large electrostatic precipitator includes a housing in which banks of vertically-extending collecting electrode plates or curtains are disposed, with particulate-laden gas passing through the housing parallel to the the plates. The particulate carried by the gas stream is charged to one polarity by means of a corona discharge, and the collecting electrode plates are oppositely charged. The charged particles are therefore electrostatically attracted to the collecting electrodes.
In order to remove the collected particulate from the collection electrodes, rapping or vibrating devices are commonly employed. In a large precipator, there are a plurality of individually controlled rappers, each rapper vibrating an electrode group comprising one or more electrode plates. Collected particulate is dislodged by the vibration and falls by gravity to a sump or the like for removal. In such a system, to prevent noticeable re-entrainment of collected particulate, it is desirable to operate only one rapper at a time. Further, it is known to be highly desirable to be able to control the rapping intensity of each individual rapper in the system. Various sections of a large precipitator tend to collect particulate at different rates. If rapping intensity higher than necessary for the actual level of particulate buildup in a particular section is employed, unnecessary stress is applied to the mechanical elements of the precipitator, leading potentially to premature failure.
To provide more meaningful and repeatable control over rapping intensity, it would also be desirable to provide an accurate indication of the rapping intensity.
A typical electrochemical rapper comprises a vertically movable plunger biased downwardly, for example by gravity, towards an impact and resting position. Preferably, the plunger rests upon an anvil rigidly connected to a group of collection electrode plates. For displacing the plunger, an electromagnetic coil is provided, which, when energized, lifts the plunger to a desired height. When the electromagnetic coil is subsequently de-energized, the plunger falls, striking the anvil and imparting vibration to the connected collection electrode plates. Rapping intensity accordingly depends upon the plunger displacement or lift before release. Plunger lift, and therefore rapping intensity, may generally be controlled by controlling the energy applied to the rapper coil.
One example of an electrostatic precipator rapper control system is disclosed in a commonly-assigned U.S. Pat. No. 3,504,480--Copcutt et al. The Copcutt et al control system generally addresses the concerns mentioned above. Power is sequentially fed to a plurality of rappers by a distribution switch. In order that the rappers may operate at different controlled intensities, power is supplied to the rappers through conduction-angle-controlled SCR's. In the Copcutt et al system, the intensity of each rapper is separately controlled.
Another electrostatic precipitor rapper control system is disclosed in the above-mentioned commonly assigned Andrews application Ser. no. 043,030. The Andrews control system, among other things, applies a short boost pulse to the rapper coil immediately prior to a lift pulse or controlled energy. During the boost pulse, full lift energy is applied for a short period of time. This boost pulse reliably gets the plunger moving, but displaces it only a relatively short distance. With the initial plunger sticking forces overcome as a result of the boost pulse, the total plunger lift or displacement accurately reflects the electrical energy applied during the subsequent lift pulse of controlled energy.
The rapper lift indicator or rapping intensity indicator of the present invention has particular advantages when employed in combination with the Andrews enhanced accuracy control system described briefly above. However, it will be appreciated that the present rapper plunger lift indicator may be employed in combination with other rapper control systems, for example, that of the Copcutt et al Pat. No. 3,504,480, with reduced accuracy if the entire energization pulse is sensed.